1. Field of the Invention
The present invention relates to a honeycomb filter.
2. Description of Related Art
One proposed honeycomb filter includes a porous partition portion, in which a cell that is open at one end and closed at the other end and a cell that is closed at one end and open at the other end are alternately disposed, and a layer for trapping and removing particulate matter (hereinafter also referred to as PM) contained in an exhaust gas formed on the partition portion (see, for example, Patent Documents 1 to 3). This honeycomb filter can trap PM by the trapping layer with low pressure loss.
In another proposed honeycomb filter, a trapping layer formed on a partition portion has a decreased thickness in the central region of the honeycomb filter in the exhaust gas flow direction (see, for example, Patent Document 4). In this honeycomb filter, the permeation resistance of a partition in the central region, which generally has low exhaust gas permeability, can be decreased to increase the exhaust gas permeability, thereby increasing the amount of PM deposit in the central region. This can decrease the temperature rise in the downstream region while PM is removed by combustion.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-216226    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 6-33734    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 1-304022    [Patent Document 4] WO 2008/136232